Creationism continued as the majority view. Its adherents still embraced the notion of a divinely regulated natural order, and therefore opposed Linnaeus. In particular, Joseph Gottlieb Kölreuter (1733-1806) doggedly opposed Linnaeus' theory. In a shower of reports published over a twenty-eight-year period (1761-1788),¹ he set forth the results of hundreds of hybridization experiments.

Mayr (1982: 645) says Kölreuter's "basic objective was to prove that hybridization of two species does not produce a third species." The ultimate motive underpinning that objective was of a religious nature. Callender (1988: 44) says Kölreuter considered Linnaeus' theory "as subversive of the doctrine of Special Creation." Kölreuter's own words show he believed in a provident Nature that always took care to prevent natural hybridization:

Just as it is unlikely that two different kinds of animals living in natural freedom should ever produce a hybrid, so is it also improbable that a hybrid should arise amidst that most regular order Nature has established in the kingdom of plants. Nature, which always, even amidst the greatest seeming confusion, orders all in the most beautiful manner, has precluded such disarray. Amongst the animals, she has prevented it mainly by means of their natural instincts. Amongst the plants, where close proximity, wind, and insects all tend to produce unnatural mixing, will she no doubt have known how to counteract the disturbing effects of such forces by equally certain means. Probably it will be found that those means include natural instincts, just as in the case of animals.²

Kölreuter's objections. Kölreuter claimed two phenomena refuted Linnaeus' theory.³ First, he had noticed that some hybrids, when self-fertilized, produce progeny more similar to one or the other of their original parental forms than to the hybrid itself. On this basis he surmised (but did not directly observe) that later-generation hybrids would eventually become perfectly similar to the original forms. In subsequent literature this supposed phenomenon was called "reversion." Modern research has shown that while some hybrids, from some crosses, do exhibit such a tendency, there is no rule of the rigid sort Kölreuter alleged. Some hybrids show such a tendency. Others don't.

Second, he had observed that when the hybrids derived from a cross are mated in each subsequent generation, not with each other, but always with one of the two parental forms originally producing them, they became more and more like that form with each succeeding generation. After many generations, they became indistinguishable. He called this the "transformation" of one parent into the other. Because he believed pollen derived from parental forms to be more potent than that derived from hybrids, he concluded that any naturally occurring hybrids would be subjected to an ongoing natural process that would transform them back into their parents. But this was merely a surmise, in no way based on the observation of natural populations and says nothing about hybrids produced in some other way.

In fact, as we shall see, many stable natural hybrids are known. Kölreuter's claims were, however, accepted and widely cited by naturalists who defended the notions of immutability and Special Creation.

von Gärtner(1772-1850)

Knight(1759-1838)

Lecoq(1802-1871)

Gärtner. An even more sedulous member of the conservative faction was Carl Friedrich von Gärtner (1772-1850). He, too, sought to squelch Linnaeus' heresy through the assiduous hybridization of plants. As a young man he was a close family friend of Kölreuter's and like him sought experimental support for Special Creation.⁴ In 1800, he closed shop as a medical doctor and devoted the remainder of his life to botany. From about 1824 until his death he studied plant hybrids. He eventually published the results of nearly 10,000 crossing experiments, in his Versuche und Beobachtungen über die Bastarderzeugung im Pflanzenreich (1849). Gärtner saw his work as a test of alternative hypotheses, Special Creation and Linnaeus' theory of evolution through hybridization (Callender 1988). He seems to have been driven by an awareness that many scientists accepted Linnaeus' views. As he notes himself in the introduction to his book,

Hybridization is still [i.e., in 1849] considered by many botanists, to be part of nature's design and purpose, especially by those who believe that species-rich genera only could have come into being via hybridization, such as [Thomas Andrew] Knight, W. Herbert, H. Lecoq, L. Reichenbach, and many others.... Kölreuter has already disputed this hypothesis, and it will be shown here, as our investigations concerning the nature of hybridization are presented, that the essential nature of the pure species contradicts this claim.⁵

The men he mentions were all leading botanists of the era. Thomas Andrew Knight was long president of the London Horticultural Society (later the Royal Horticultural Society) from 1811 to 1838 and is generally considered the most distinguished horticulturist of his day. It is no surprise he believed hybridization could produce new forms. He had developed many new types of fruit by such means himself. In the first half of the nineteenth century, William Herbert (1778-1847) was the foremost English authority on plant hybridization.⁶ Among botanists, he was renowned as the author of the monumental Amaryllidaceae (1837). Ludwig Reichenbach⁷ and Henri Lecoq⁸ were both prolific authors of botanical literature and directors of important scientific institutions. Olby (1997) argues even Gregor Mendel's

research in hybridization was in the tradition of Linnean botany which accepted that the species we know today are the children of the crossing of fewer primordial forms.

(This certainly isn't the Mendel we hear about in biology and genetics classes!) Thus, Linnaeus' theory was still taken quite seriously a century after he had conceived it. Even Mendel (1822-1884), a younger man than Darwin, was apparently influenced by it.

Darwin, Creationism, and Hybrid Sterility. Darwin, of course, labored long and hard against creationism and the idea of immutability. So he did his best to undermine the idea that God specially endows hybrids with sterility. In the eighth chapter of the Origin, he argues that the complexity and inconsistency of the rules of hybridization speak against the idea that hybrid sterility is God's way of preventing the blending of distinct types of organisms. After discussing available data on hybridization, he writes (1859: 260):

Now do these complex and singular rules indicate that species have been endowed with sterility simply to prevent their becoming confounded in nature? I think not. For why should the sterility be so extremely different in degree, when various species are crossed, all of which we must suppose it would be equally important to keep from blending together? Why should the degree of sterility be innately variable in the individuals of the same species? Why should some species cross with facility, and yet produce very sterile hybrids; and other species cross with extreme difficulty, and yet produce fairly fertile hybrids? Why should there often be so great a difference in the result of a reciprocal cross between the same two species? Why, it may even be asked, has the production of hybrids been permitted?

Like other naturalists of the time, Darwin took this issue quite seriously -- elsewhere he commented that "if it could be proved or rendered highly probable that sterility in the first cross or in the hybrid offspring was a specially created endowment, it would be to us a fatal difficulty."⁹ However, in his theory Darwin did not pursue the implications of the fact that many hybrids are not sterile. He merely used the known facts about hybridization, in particular the fact that different types of hybrids display different degrees of fertility, to undermine creationism. Giving scant attention to hybridization as a means of producing new types of organisms, he largely ignored Linnaeus' claims that evolution happened in that way. As a result, the ascendancy of his theory put such ideas in the shade. There they remained for the next forty years, although a few scientists (e.g., Kerner 1871, 1894-1895) did continue to argue that hybridization could produce new types of organisms. Not until Hugo de Vries (1901, 1903) published his theories did such ideas again reach the forefront of evolutionary debate.

Modern neo-Darwinism has retained creationism's old idea of insurmountable barriers even while discarding the allied notion of immutability. It envisions new types of organisms arising via a process in which they gradually change. But that change is supposed to occur only within isolated populations, ones that do not interbreed. Under that view, barriers preventing interbreeding are presumed to be necessary if populations are to differentiate, by gradually accumulating differences, and not blend back together. This view ignores the great mass of evidence demonstrating that a wide variety of organisms produce hybrids in a natural setting, that such hybrids occur in large numbers, and that they often are not sterile. A very small portion of this evidence will be discussed in subsequent sections.